This invention relates to radiotelephone systems and methods, and more particularly to satellite radiotelephone systems and methods.
Satellite radiotelephone systems are being developed and deployed at many locations around the world. As is known to those having skill in the art, a satellite radiotelephone system generally includes at least one satellite and at least one gateway that interfaces the satellite radiotelephone system to other telephone systems, such as wire telephone systems and/or cellular radiotelephone systems. A plurality of user terminals communicate with the at least one satellite, to provide satellite communications. The user terminals may be mobile or fixed. It will be understood that the user terminal may be a satellite radiotelephone, a combined cellular and satellite radiotelephone, a high functionality terminal including Personal Communications Systems (PCS) terminals and/or a portable computer with a satellite radiotelephone modem. The basic principles regarding a satellite radiotelephone system are described in the publication entitled Dual-Mode Cellular/Satellite Hand-Held Phone Technology by coinventor Karabinis et al., WESCON/96, pp. 206-222, Oct. 22, 1996, and need not be described in further detail herein.
In geostationary systems, or non-geostationary systems such as Low Earth Orbit (LEO) or Medium Earth Orbit (MEO) systems, a user terminal can communicate with more than one satellite. Accordingly, satellite diversity may be provided so that the user terminal communicates with at least two satellites, to thereby allow reduced shadowing and/or blockage problems. In particular, many of the obstructions in the direct line of sight do not completely block the path, but rather attenuate the signal. Given two or more satellites in view at the same time, the probability of signal blockage or shadowing to all satellites may be significantly reduced. In the forward link or downlink, satellite diversity may provide substantial link availability benefit if one of the satellites is completely obstructed. With multipath fading, forward link diversity can provide great benefits such as a 3 dB increase in received energy plus diversity gain.
It is known to achieve forward link diversity operation by allowing a user terminal in a downlink region to receive two bursts per frame from the best two visible satellites in two different Time Division Multiple Access (TDMA) time slots and two different carrier frequencies. The frequencies may be assigned based on the location of the user terminals, for example using a Region Oriented Frequency Assignment (ROFA) scheme.
When transmitting two bursts per frame for a user terminal, it generally is important for the user terminal to have knowledge of when to receive the two bursts per frame. It is desirable for the user terminal to know when to receive these bursts so that the bursts may be accurately received without the need to consume excessive power.
It is known to provide forward link diversity by dividing the user terminals into three groups that receive downlink signal bursts from a plurality of satellites transmitting TDMA signals. A first satellite first transmits to a first group of user terminals using a first number of time slots in a TDMA frame. In addition, the first satellite and a second satellite transmit in alternate TDMA frames to a second group of user terminals using a second number of time slots in the TDMA frame period. Finally, the second satellite transmits to a third group of user terminals using a third number of time slots in the TDMA frame period. Unfortunately, this scheme may only allow forward link diversity operation to the second group of user terminals. See for example U.S. Pat. No. 5,841,766 to Dent et al. entitled Diversity-Oriented Channel Allocation In A Mobile Communications System that is assigned to the assignee of the present application, the disclosure of which is hereby incorporated herein by reference in its entirety.
Another known solution is based upon the concept that the time slots are distributed along the hyperbola of constant differential time delay to two satellites and the carrier frequency usage is distributed along orthogonal lines. See the above-cited Dent et al. patent. Unfortunately, this approach may place restrictions with regard to the distribution of user terminals. In particular, this approach may only be applicable if the selected satellites are from one orbit so that the user terminals that are located on the contour of constant time delay will be orthogonal to the carrier frequency usage. This may not always be the case with Low Earth Orbit (LEO) or Medium Earth Orbit (MEO) systems.
Accordingly, notwithstanding the above-described improvements, there continues to be a desire to provide improved forward link diversity for satellite radiotelephone systems. There also is a need for forward link diversity systems and methods that need not place a restriction on the distribution of the user terminals in any particular region. These forward link diversity systems and methods also preferably should not restrict the selection of the best visible satellites serving the user terminals.
The present invention can determine a proper time of reception of two successive forward link signal bursts for user terminals that are randomly distributed in a downlink region based on the separation distance of the user terminal from the Time Alignment Center (TAC) of the downlink region and the position of the satellites. Accordingly, forward link diversity may be accomplished without the need to restrict the distribution of the user terminals in a downlink region. There also need be no restriction in selecting the best two visible satellites serving the user terminals. There may be a restriction in the size of the downlink region which can be based on the desired capacity for that region. Improved forward link diversity systems and methods thereby may be provided.
More specifically, the present invention can transmit downlink signal bursts from a plurality of visible satellites to a plurality of user terminals in a downlink region that is serviced by the plurality of visible satellites. A gateway transmits such that the diversity bursts are received by a user terminal at the TAC one-half a frame apart. A first downlink signal burst is sent from a first one of the visible satellites to a first user terminal in the downlink region. A second downlink signal burst is sent from a second one of the visible satellites to the first user terminal at a time that is a function of a separation distance of the first user terminal in the downlink region from the TAC of the downlink region. First downlink signal bursts and second downlink signal bursts also are sent to other user terminals in the downlink region. The first and second downlink signal bursts preferably are sent over respective first and second carrier frequencies.
In preferred embodiments, the downlink signal bursts are transmitted in a plurality of repeating frames of downlink signal bursts of a predetermined frame repetition period. The second downlink signal burst is sent at a time that is one-half the predetermined frame repetition period plus a delay time xcex94T that is a function of a separation distance of the first user terminal in the downlink region from the TAC of the downlink region. The delay time xcex94T preferably is determined by the differential propagation delay of the second downlink signal burst from the second one of the visible satellites to the first user terminal and to a user terminal that is located at the TAC, minus a differential propagation delay of the first downlink signal burst from the first one of the visible satellites to the first user terminal and to a user terminal that is located at the TAC. Accordingly, the timing of forward link signal bursts may be determined for diversity operation.
As was described above, the downlink region preferably is restricted in size based on the desired capacity for the downlink region. In particular, the downlink region preferably is sufficiently small such that the first user terminal can receive the first downlink signal burst from the first visible satellite and the second downlink signal burst from the second visible satellite when the first and second visible satellites are located on respective opposite sides of the horizon and the first user terminal is located in the downlink region farthest from the TAC, with sufficient time therebetween that the first user terminal can transmit an uplink signal burst between the first and second downlink signal bursts. The present invention also need not restrict selection of the best two visible satellites serving the user terminals. Thus, the first satellite preferably is at a highest elevation angle and the second satellite preferably is at a next highest elevation angle. However, other satellites may be used.
An uplink or return signal burst also preferably is transmitted from the first user terminal for reception by at least two of the plurality of visible satellites to thereby provide return link diversity. Preferably, the uplink signal burst is transmitted a guard time after an immediately proceeding uplink signal burst from an immediately proceeding one of the user terminals. The guard time preferably is based upon a time of arrival difference for the adjacent uplink signal bursts to one of the plurality of visible satellites.
A fixed guard time or a variable guard time may be used. The fixed guard time preferably corresponds to a maximum time of arrival difference between adjacent uplink signal bursts from a pair of user terminals that are a maximum distance apart to one of the plurality of visible satellites. The variable guard time preferably corresponds to a time of arrival difference between adjacent uplink signal bursts from a corresponding pair of user terminals to one of a plurality of visible satellites.
Downlink signal bursts may be received from a plurality of visible satellites at a user terminal that is located at any arbitrary position in a downlink region that is serviced by the plurality of visible satellites. A first downlink signal burst is received from a first one of the visible satellites at the user terminal that is located at any arbitrary position in the downlink region. A second downlink signal burst is received from a second one of the visible satellites at the user terminal that is located at any arbitrary position in the downlink region at a time that is a function of a separation distance of the arbitrary position of the user terminal in the downlink region from a Time Alignment Center (TAC) of the downlink region. The first downlink signal burst may be received over a first carrier frequency and the second downlink signal burst may be received over a second carrier frequency. The size of the downlink region may be limited as was described above. Uplink signal bursts also may be transmitted as was described above. The received first and second downlink signal bursts may be diversity combined at the user terminal.
Satellite radiotelephone systems according to the present invention communicate with a plurality of user terminals in a downlink region. The satellite radiotelephone system preferably includes a first satellite that sends a first downlink signal burst to each of the plurality of user terminals in the downlink region. A second satellite sends a second downlink signal burst to each of the plurality of user terminals to be received at a respective time that is a function of the separation distance of the respective user terminal in the downlink region from the TAC of the downlink region. The downlink time for the second downlink signal burst may be determined as was described above. The size of the downlink region may be limited in the manner that was described above. Uplink signal bursts also may be received in the manner that was described above.
Finally, satellite user terminals according to the present invention may be located at any arbitrary position in a downlink region that is serviced by a plurality of visible satellites. The satellite user terminals include a receiver that receives a first downlink signal burst from a first one of the visible satellites and that receives a second downlink signal burst from a second one of the visible satellites. A diversity combiner combines the received first and second downlink signal bursts. The first downlink signal burst preferably is received over a first carrier frequency and the second downlink signal burst preferably is received over a second carrier frequency. The downlink region may be sized as was described above. The user terminal may include a transmitter that transmits an uplink signal burst using guard times as was described above. Accordingly, satellite radiotelephone systems, methods and user terminals may be provided that can be randomly distributed in a downlink region, and that can include forward link diversity and return link diversity.